Conventionally, in the field of optical communication or optical measurement, various optical control devices such as a waveguide type optical modulator and a waveguide type optical switch where an optical waveguide and a control electrode are formed on a substrate having an electro-optical effect have become commercially available. Many of the optical control devices which are currently being used are configured to include an optical waveguide 2, a signal electrode 4 and a ground electrode 5 which are formed on an electro-optical crystal substrate 1 having a thickness of about 0.5 to 1 mm, as illustrated in FIG. 1. FIG. 1 illustrates an example of an optical modulator that uses a Z-cut LiNbO3 substrate, in which reference numeral 3 indicates a buffer layer such as SiO2 film.
Specifically, in the waveguide type optical modulator, a microwave signal is applied to the control electrode in order to control and modulate an optical wave propagating through the optical waveguide. Therefore, there is a need for achieving an impedance matching between the control electrode in the optical modulator and a signal path, such as a coaxial cable which introduces microwaves into the optical modulator, in order to allow the microwave to propagate effectively in the control electrode. For this reason, as illustrated in FIG. 1, a type of control electrode where a signal electrode 4 is interposed between ground electrodes 5 so that a so-called coplanar control electrode has been used.
However, in the case of the coplanar control electrode, since an external electric field does not operate efficiently in a direction (corresponding to a vertical direction in the case of the Z-cut LiNbO3 substrate illustrated in FIG. 1) where efficiency of the electro-optical effect of the substrate 1 is high, a larger voltage is required in order to obtain a required optical modulation degree. Concretely, when a LiNbO3 substrate is used and an electrode length along the optical waveguide is 1 cm, a half-wavelength voltage of about 10 to 15 V is required.
As illustrated in FIG. 2, Patent Document 1 discloses a configuration in which the optical waveguide is formed of a ridged waveguide 20, and the ground electrodes 5, 51, and 52 are disposed closer to the signal electrodes 4 and 41 in order to enhance an optical confinement factor of the optical waveguide and to more efficiently apply an electric field generated by the control electrode to the optical waveguide. According to this configuration, it is possible to realize a reduction in driving voltage to some degree but it is essential to reduce the driving voltage much more in order to realize a high-speed modulation in a high frequency band.
In addition, as illustrated in FIG. 3, Patent Document 2 discloses that the substrate is interposed between the control electrodes, and the electric field is applied in a direction (corresponding to a vertical direction in the case of the Z-cut LiNbO3 substrate illustrated in FIG. 3) where the efficiency of the electro-optical effect is high. Moreover, the optical modulator illustrated in FIG. 3 polarizes reversely the substrate having the electro-optical effect, and forms substrate regions 10A and 10B in which the respective spontaneous polarization directions (the directions of arrows in FIG. 3) are different from each other, and the optical waveguide 2 is formed in each of the substrate regions. When the electric field is applied to each optical waveguide by the common signal electrode 42 and the ground electrode 53, it is possible to generate a phase change for each optical waveguide such that the phase change of one optical waveguide is different from that of another optical waveguide. Using this differential driving, it is possible to reduce the driving voltage much more.
However, in the electrode structure illustrated in FIG. 3, a refractive index of the microwave becomes high, and thus it is difficult to realize a velocity matching between the optical wave which is propagating through the optical waveguide and the microwave which is a modulation signal. Moreover, since the impedance is reduced to the contrary, there is a drawback in that it is difficult to achieve the impedance matching with the signal path of the microwave.
On the other hand, in the following Patent Document 3 or 4, the optical waveguide and a modulating electrode are formed integrally in a very thin substrate which has a thickness of 30 μm or less (the substrate will be referred to as a “sheet-like substrate,” and another substrate which has a lower dielectric constant than that of the sheet-like substrate is bonded to the sheet-like substrate, thereby lowering an effective refractive index with respect to the microwave and achieving the velocity matching between the microwave and the optical wave.
However, even when the control electrode having the structure as illustrated in FIGS. 1 to 3 is formed in the optical modulator that uses such a sheet-like substrate, the above-mentioned problems still have fundamentally not been resolved. When the substrate is interposed between the control electrodes illustrated in FIG. 3, the refractive index of the microwave tends to decrease if the thickness of the substrate is thin, but it is difficult to realize the velocity matching between the optical wave and the microwave. Even when it depends on the width of the electrode, when a sheet-like substrate made of LN is used for example, the effective refractive index is about 5, which is far lower than an optimal value of 2.14. On the other hand, the impedance tends to decrease as the substrate becomes thinner, which causes a mismatching in impedance to be large.    Patent Document 1: U.S. Pat. No. 6,580,843    Patent Document 2: Japanese Patent No. 3638300    Patent Document 3: JP-A 64-018121 (KOKAI)    Patent Document 4: JP-A 2003-215519 (KOKAI)    Patent Document 5: JP-A 6-289341 (KOKAI)